That simply isn't true, sometimes you cannot reload the weapon at all, regardless of context in the game. You simply cannot reload it, I've had this happen to me numerous times, and it happens when there is no clip in the gun at all. Let's say I'm taking a clip out of my inventory to load into my gun, why would that need space?

I cannot find a place to submit a report but I'd like to say that generally if not always the magazine problems only occur after having switched servers. I'd be willing to wager you can reproduce it by "server hopping".